1. Field of the Invention
The present invention relates to a novel in-line cell useful in absorption spectroscopy. The present invention also relates to a system for performing an absorption spectroscopy measurement in a sample and to a semiconductor processing apparatus which comprise the novel in-line cell.
2. Description of the Related Art
Semiconductor integrated circuits (ICs) are manufactured by a series of processes, many of which involve the use of gaseous materials. Included among such processes are etching, diffusion, chemical vapor deposition (CVD), ion implantation, sputtering and rapid thermal processing.
Many of the gases used in these processes are highly reactive and tend to form deposits on surfaces with which they come into contact, especially under conditions used in IC fabrication, such as high temperature or plasma conditions. When an in-line spectroscopic sensor is used to monitor a process in such aggressive atmospheres, deposits from the process gases tend to form on various surfaces, for example, the optical surfaces, of the sensor. As a result, sensor performance tends to deteriorate.
The sensitivity of detection of gas phase molecular species by absorption spectroscopy increases as the length of the light path through the sample increases, for constant pressure and concentration. The intensity of light reaching the detector is given by Beer's Law as follows: EQU I=I.sub.o .multidot.e.sup.-.alpha. cl
where I.sub.o is the intensity of the incident radiation, .alpha. is the absorptivity, l is the pathlength through the sample, c is the concentration of the impurity in the sample (by volume), and P is the total pressure of the sample. For small absorptions, the amount of light absorbed is given by EQU I-I.sub.o =.alpha.cl
In order to make l large, it is frequently impractical to place the light source and detector very far apart and so "folded" light paths are often used, in which mirrors reflect the light back and forth through the sample gas many times.
The Herriott design is often preferred for tunable diode laser absorption spectroscopy (TDLAS). As shown in FIG. 1, the Herriott cell 100 uses two curved mirrors 102 mounted at opposite ends of a usually cylindrical gas sample cell 104. Simple multi-pass arrangements are often used, such as described in U.S. Pat. No. 3,524,066, to Blakkan, and U.S. Pat. No. 5,173,749, to Tell et al, the contents of which are herein incorporated by reference. A planar polygonal multipass cell is described by the present inventors in copending application Ser. No. 08/711,504, filed Sep. 10, 1996 now U.S. Pat. No. 5,818,578, the contents of which are herein incorporated by reference.
In the multipass cells described above, deposits formed on optical components thereof can adversely impact on the sensitivity and operation of the sensor. For example, deposits formed on the reflective surfaces of the mirrors can reduce their reflectivity and hence the light intensity which reaches the detector after multiple reflections of the light beam. Likewise, the formation of deposits on the light transmissive window(s), through which the light beam enters and exits the measurement cell, acts to reduce the light intensity reaching the detector. Such reduction in light intensity reduces the measurement sensitivity and may eventually lead to a condition in which the sensor does not function at all.
Deposits on the mirrors and light transmissive windows can be removed by disassembling the sensor and mechanically cleaning the contaminated components. Such maintenance, however, is inconvenient and expensive. Avoidance thereof is, therefore, desirable.
The use of a purge gas to minimize deposits on optical surfaces in an in-situ particle monitor has been described in U.S. Pat. No. 5,360,980, to Borden. As disclosed therein the purge gas flow should be large compared with the total process gas flow. This requires that the vacuum pump used to evacuate the process chamber be increased in size. Such requirement is both expensive and impractical.
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel in-line cell useful in absorption spectroscopy. The in-line cell allows for accurate, in-situ absorption spectroscopy measurements which can be used, for example, to accurately and sensitively measure the concentration of gas phase molecular impurities in a sample. The problems associated with the formation of deposits on optical surfaces of the cell, such as the reflective surfaces of mirrors and the light transmissive window surfaces within the measurement cell can be avoided or conspicuously ameliorated by the inventive cell.
It is a further object of the present invention to provide an absorption spectroscopy system which includes the inventive in-line cell.
It is further an object of the present invention to provide a semiconductor processing apparatus which includes the absorption spectroscopy system for performing in-situ measurements.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.